get some columns of a table only have null values - oracle

Question

I need to know what columns of one table have only null values. I understand that I should do a loop in user_tab_columns. But how detect only columns with null value?
Thanks and sorry for my English

Answer 1

To perform a query where you don't know the column identifies in advance, you need to use dynamic SQL. Assuming you already know the table is not empty, you could do something like:
declare
l_count pls_integer;
begin
for r in (
select table_name, column_name
from user_tab_columns
where table_name = 'T42'
and nullable = 'Y'
)
loop
execute immediate 'select count(*) '
|| ' from "' || r.table_name || '"'
|| ' where "' || r.column_name || '" is not null'
into l_count;
if l_count = 0 then
dbms_output.put_line('Table ' || r.table_name
|| ' column ' || r.column_name || ' only has nulls');
end if;
end loop;
end;
/
Remember to set serveroutput on or your client's equivalent before executing.
The cursor gets the columns from the table which are declared as nullable (if they aren't, not much point checking them; though this won't catch explicit check constraints). For each column it builds a query to count the rows where that column is not null. If that count is zero then it didn't find any that are not null, therefore they all are. Again, assuming you know the table isn't empty before you start.
I've included the table name in the cursor select list and references so you only need to change the name in one place to search a different table, or you could use a variable for that name. Or check multiple tables at once by changing that filter.
You may get better performance by selecting a dummy value from any non-null row, with a rownum stop check - which means it will stop as soon as it finds a non-null value, rather than having to check every row to get an actual count:
declare
l_flag pls_integer;
begin
for r in (
select table_name, column_name
from user_tab_columns
where table_name = 'T42'
and nullable = 'Y'
)
loop
begin -- inner block to allow exception trapping within loop
execute immediate 'select 42 '
|| ' from "' || r.table_name || '"'
|| ' where "' || r.column_name || '" is not null'
|| ' and rownum < 2'
into l_flag;
-- if this foudn anything there is a non-null value
exception
when no_data_found then
dbms_output.put_line('Table ' || r.table_name
|| ' column ' || r.column_name || ' only has nulls');
end;
end loop;
end;
/
or you could do something similar with an exists() check.
If you don't know that the table has data then you can do a simple count(*) from the table before the loop to check if it is empty, and report that instead:
...
begin
if l_count = 0 then
dbms_output.put_line('Table is empty');
return;
end if;
...
Or you could combine it with the cursor query, but this would need some work if you wanted to check multiple tables at once as it would stop as soon as it found any empty one (have to leave you something to do... *8-)
declare
l_count_any pls_integer;
l_count_not_null pls_integer;
begin
for r in (
select table_name, column_name
from user_tab_columns
where table_name = 'T42'
and nullable = 'Y'
)
loop
execute immediate 'select count(*),'
|| ' count(case when "' || r.column_name || '" is not null then 1 end)'
|| ' from "' || r.table_name || '"'
into l_count_any, l_count_not_null;
if l_count_any = 0 then
dbms_output.put_line('Table ' || r.table_name || ' is empty');
exit; -- only report once
elsif l_count_not_null = 0 then
dbms_output.put_line('Table ' || r.table_name
|| ' column ' || r.column_name || ' only has nulls');
end if;
end loop;
end;
/
You could of course populate a collection or make it a pipelined function or whatever if you didn't want to reply on dbms_output, but I assume this is a one-off check so it is probably acceptable.

Answer 2

You can loop though your columns and count null rows. If it is same with your table count, then that column has only null values.

Answer 3

The first question is: one column with zero row could be regarded as only (null) value containing column. But it can remain your decision: the scripts below provide solutions to both ways. (In my opinion: no. The empty columns is not a column with only (null) value)
If you want to know the (null) values about one table you can it with count(column):
select count(column) from table
and when the count(column) = 0 then the column has only (null) value or has no value. (So, you cannot make a correct decision).
E.g. The following three tables (x, y and z) has the following columns:
select * from x;
N_X M_X
---------------
100 (null)
200 (null)
300 (null)
select * from y;
N_Y M_Y
---------------
101 (null)
202 (null)
303 apple
select * from z;
N_Z M_Z
---------------
The count() selects:
select count(n_x), count(m_x) from x;
COUNT(N_X) COUNT(M_X)
-----------------------
3 0
select count(n_y), count(m_y) from y;
COUNT(N_Y) COUNT(M_Y)
-----------------------
3 1
select count(n_z), count(m_Z) from z;
COUNT(N_Z) COUNT(M_Z)
-----------------------
0 0
As you can see, the difference between x and y is appears but you cannot decide that the table z has no rows or only full of (null) values.
The general solution:
I have separeted the schema and the db level but the basic idea is common:
Schema level: the current user’s table
DB level: all users or a chosen schema
The number of (null) in one columns:
all_tab_columns.num_nulls
(Or: user_tab_columns, num_nulls).
And we need the num_rows of the table:
all_all_tables.num_rows
(Or: user_all_tables.num_rows)
Where the num_nulls equals to num_rows there are only (null) values.
Firstly, you need to run the DBMS_STATS for refresh the statistics.
on database level:
exec DBMS_STATS.GATHER_DATABASE_STATS;
(it can use a lot of resources)
on schema level:
EXEC DBMS_STATS.gather_schema_stats('TRANEE',DBMS_STATS.AUTO_SAMPLE_SIZE); (owner = tranee)
-- column with zero row = column has only (null) values -> exclude num_nulls > 0 condition
-- column with zero row <> column has only (null) values -> include num_nulls > 0 condition
the scripts:
-- 1. current user
select
a.table_name,
a.column_name,
a.num_nulls,
b.num_rows
from user_tab_columns a, user_all_tables b
where a.table_name = b.table_name
and num_nulls = num_rows
and num_nulls > 0;
-- 2. chosen user / all user -> exclude the a.owner = 'TRANEE' condition
select
a.owner,
a.table_name,
a.column_name,
a.num_nulls,
b.num_rows
from all_tab_columns a, all_all_tables b
where a.owner = b.owner
and a.table_name = b.table_name
and a.owner = 'TRANEE'
and num_nulls = num_rows
and num_nulls > 0;
TABLE_NAME COLUMN_NAME NUM_NULLS NUM_ROWS
----------------------------------------------------
LEADERS COMM 4 4
EMP_ACTION ACTION 12 12
X M_X 3 3
These tables and columns have only (null) values in tranee schema.